Diphtheria is an infection caused by the Corynebacterium and is spread via air and direct contact. The symptoms often present gradually over a couple of days with a sore throat and fever. In severe cases, a grey or white patch develops in the throat, which can block the airway and create a barking cough. Diphtheria can also cause a swollen neck, sometimes referred to as a bull neck. The disease is fatal in between 5% and 10% of cases. In children under five years old and adults over 40 years, the fatality rate may be as much as 20%

Diphtheria was first described in the 5th century BC by Hippocrates, but it was not until 1883, that Edwin Klebs identified the bacterium.

In 1897, Paul Ehrlich developed a standardised unit of measure for diphtheria antitoxin. This was the first ever standardisation of a biological product, and played an important role in future developmental work on sera and vaccines.

A diphtheria vaccine is effective for prevention and available in a number of formulations. Three or four doses, given along with tetanus vaccine and pertussis vaccine, is recommended  during childhood. The vaccine should be renewed every ten years.

Before the vaccine became part of the Danish national child vaccine program, recurrent epidemics of Diphtheria were observed. The last case of diphtheria in Denmark was discovered in December 1988, but outbreaks are still seen in countries with low vaccination rate. There is general consensus among scientists that the almost complete eradication of the disease in western countries is due to the vaccine programs.